Update CHANGELOG [ci skip]

CHANGELOG.md

-v 1.5.0 (unreleased)
+v 1.5.0
+- Update vendored toml !258
+- Release armel instead arm for Debian packages !264
+- Improve concurrency of docker+machine executor !254 
+- Use .xz for prebuilt docker images to reduce binary size and provisioning speed of Docker Engines !249
+- Remove vendored test files !271
+- Update gitlab-runner-service to return 1 when no Host or PORT is defined !253
+- Log caching URL address
+- Retry executor preparation to reduce system failures !244
+- Fix missing entrypoint script in alpine Dockerfile !248
+- Suppress all but the first warning of a given type when extracting a ZIP file !261
+- Mount /builds folder to all services when used with Docker Executor !272
+- Cache docker client instances to avoid a file descriptor leak !260
+- Support bind mount of `/builds` folder !193
 
 v 1.4.2
 - Fix abort mechanism when patching trace